Did this error just start appearing on a good working machine, or has something recently been done like game eproms removed/changed, jumpers on boards changed, etc? If there was any work done to the machine before this happened we need to know what was done.
Are there any other indications? When the machine is powered up does it give the usual "bong" sound from the speaker a few seconds after power up? You can check the 12 volt power supply in the top box, that is what powers the Dotmation controller board (inside the metal box up there) and the dot matrix screen that mounts to the metal box. And make sure the ribbon cable is connected good to the dot controller board on the left side of the dot controller.
Just to make sure, this is a WMS 40X Stampede with 3 mechanical spinning reels, right?